Uranyl minerals crystallization

In some ways, the skeletal behavior of uranium is quantitatively similar to that of alkaline earths. It is known that the uranyl ion (U02 ) exchanges with Ca + on the surfaces of bone mineral crystals, although it does not participate in crystal formation or enter existing crystals. The early distribution of uranium in different parts of the skeleton is similar to that of calcium. Uranium initially deposits on all bone surfaces but is most highly concentrated in areas of growth. Depending on the microscopic structure of the bone of each species, uranium on bone surfaces may gradually diffuse into bone volume such... 

The crystal chemistry of phosphate minerals has recently been reviewed [9, 10]. These references present a stmctural hierarchy based on the pol5mierization of polyhedra of higher bond-valence, especially tetrahedra and octahedra. In a similar fashion, an extensive stmctural hierarchy of uranyl minerals and inorganic compounds has been developed over the last decade [11, 12]. This chapter follows the concepts and principles of both of these stmctural hierarchies, but places the primary emphasis on actinide coordination. As the coordination environments of the actinides differ with valence state [13, 14], it has been found convenient to discuss the compounds of the lower valence-state actinides separately from those of the higher valence-states. 

The order of presentation of the uranium minerals will follow chemical groups. The U minerals are discussed first, followed by the niobates, tantalates and titanates. These two groups include the primary reduced minerals. The uranyl minerals are considered in the order hydrated oxides, silicates, phosphates and arsenates, vanadates, molybdates, sulphates, carbonates, and selenates and tellurates. Each section includes an evaluation of the known crystal chemistry and its effect on chemical variability and occurrence of mineral species. 

The uranyl minerals are considered in groups, depending on their associated anion. This approach is useful because each of these groups has many characteristics in common, including those of occurrence and crystal chemistry. Within each group sub-classification by UO2 XO ratios leads to interesting comparisons and some very specific mineral families. 

This group of uranyl minerals occurs almost exclusively in alteration haloes on uraninite in association with the uranyl oxide hydrates. The Pb minerals are common because of the available radiogenic Pb especially in geologically older deposits. The phases are usually very fine-grained and intimately intergrown with other minerals or with one another. Only rarely do they form as recognizable small crystals. Colour can be a guide to specific mineral identifications, but X-ray diffrac-... 

Keywords Aqueous speciation Bond valence Borate minerals Crystallization Dissolution Morphology Surface structure Uranyl minerals... 

The crystal structures of uranyl minerals are dominated by the structure of the complex uranyl cation (1102) ". The central U is coordinated by two anions... 

The known uranium(VI) carbonate soHds have empirical formulas, 1102(003), M2U02(C03)2, and M4U02(C03)3. The soHd of composition 1102(003) is a well-known mineral, mtherfordine, and its stmcture has been determined from crystals of both the natural mineral and synthetic samples. Rutherfordine is a layered soHd in which the local coordination environment of the uranyl ion consists of a hexagonal bipyramidal arrangement of oxygen atoms with the uranyl units perpendicular to the orthorhombic plane. Each uranium atom forms six equatorial bonds with the oxygen atoms of four carbonate ligands, two in a bidentate manner and two in a monodentate manner. 

Appendix C contains the chemical formulae for the minerals used in this book. There are very few minerals that have the ideal crystalline structures discussed above. There are sufficient substitutional impurities, crystal defects, and distortions that make the CBPC structure significantly different from the models discussed above. Several well-established minerals exhibit these features, as are many of those listed in Appendix C. For example, Ca(UO2)2(PO4)2T0H2O is formed by the substitution of Ca in autunite by uranyl (UO2) ions, making the autunite a mineral of radioactive uranium. Similarly, (Ce,Th)P04 is formed by the substitution of the Ce in monazite by Th. Numerous minerals can be formed by substitutions and provide a researcher sufficient degree of freedom to synthesize very complex minerals to produce useful CBPCs. 

Single crystal structure X-ray diffraction analyses and structural classification of synthetic and natural mineral phases have revealed interesting actinide coordination chemistry. " This approach has led to the identification of in CaU(U02)2(C03)04(0H)(H20)7, the mineral wyartite. The structure contains three unique U positions. Two of these are uranyl ions with the typical pentagonal-bipyramidal coordination. The third is also seven-coordinate, but does not contain -yL oxygens and polyhedral geometry and electroneutrality requirements indicate that this site contains U. ... 

Figure 50 Uranyl carbonate layers in the crystal structure of the mineral rutherfordine, UO2CO3 (Finch, Cooper et al. Cannadian Mineralogist 1999, 37, 929-938).

The hexavalent-uranium phosphate minerals (Table 14) are important and widespread uranyl-oxysalt minerals. Their structures and behavior are dominated by the crystal chemistry of the (U 02) uranyl group they have been described in detail by Burns (1999) and will not be considered any further here. 

Piret P, Piret-Meunier J (1988) Nouvelle determination de la structure cristalline de la dumontite Pb2((U02)302(P04)3)(H20)5) Bull Mineral 111 43 9-442 Piret P, Piret-Meunier J, Declercq JP (1979) Structure of phuralumite. Acta Crystallogr B35 1880-1882 Piret P, Deliens M, Piret-Meunier J (1985) Occurrence and crystal structure of kipushite, a new copper-zinc phosphate from Kipushi, Zaire. Can Mineral 23 35-42 Piret P, Deliens M, Piret-Meunier J (1988) La francoisite-(Nd), nouveau phosphate d uranyle et de terres rares proprietes et structure crystalline. Bull Mineral 111 443-449 Piret P, Piret-Meunier J, Deliens M (1990) Composition chimique et structure cristalline de la dewindtite Pb3(H(U02)302(P04)2)2-12H20. Eur J Mineral 2 399-405... 

Bright orange-yellow, very hygroscopic, amorphous powder or hexagonal crystals d 7.368. Soluble in mineral acids, forming uranyl salts. In water, it hydrates in 24 hours at room temperature to give UO3 H2O. 

Table 7 lists 19 known minerals that may be classified as alkali or alkaline-earth uranyl oxide hydrates. They have often been referred to as uranates as well as uranyl oxides, but as details of their crystal structures become known it is apparent that they are closely related to the uranyl oxides described above. In fact, Sobry and Noe-Spirlet and Sobry" have shown that substitutional series exist between schoepite and all the minerals on the list. It was proposed that the minerals can be explained by the general formula... 

Chemically, the uranyl silicates form three groups depending on the uranium/silicon ratio. The most populated group, the 1 1 group, is one of the best studied. Stohl and Smith and Sidorenko and co-workers reviewed the crystal chemistry of these minerals. They showed that all 1 1 minerals have essentially the same basic structural unit [(U02)Si04]J" , an infinite chain of edge-shared uranyl pentagonal dipyramids and silicate... 

The uranyl vanadates form mineral groups distinct from the phosphates and arsenates because of the markedly different chemistry of the vanadium ion. Like uranium, vanadium shows several valence states in nature, and its detailed mineralogy is very complex. The crystal chemistry of vanadium was reviewed by Evans.In its lower valence states it forms distinct vanadium minerals, but in its higher valence state 5-1- it com-... 

Christ C. L. and Clark J. R. Crystal chemical studies of some uranyl oxide hydrates. Am. Miner., 45, 1960, 1026-61. 

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